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ON ELECTROLYSIS AND ELECTRO-CHEMISTRY. 195 
The only apparent evidence against the application of Joule’s law to 
electrolytes is the ‘innere Polarisation’ observed by Du Bois-Reymond.! * 
This phenomenon is, however, only exhibited in heterogeneous conductors, 
such as filter-paper and other porous bodies when moistened with a 
bad conductor like water. It is not shown when H,SO,, KI, or KHO is 
used, unless the porous body isa good conductor, as charcoal or cylinders 
of stiff glue containing brass filings. It may be explained by the division 
of the current between the fluid and the matrix, in the same way as the 
decomposition of AgNO; in a crack in a glass partition, observed by 
Grotthuss.? 
For the theory of the relation between E.M.F. and difference of con- 
centration of an electrolyte, see Von Helmholtz, ‘Wissensch. Abh.’ vol. 1, 
. 840. 
It is clear from what has been said abcve that the conduction of 
electricity through the electrolyte may be considered quite separately 
from the actions taking place at the electrodes. We are accordingly led 
to notice two main and almost independent divisions of the subject. The 
first of these relates to the transformations of energy incidental to, and 
represented by, the separation of ions, the secondary actions, the thermo- 
electric effects, the electromotive forces of polarisation at the electrodes. 
This part may becalled the thermodynamics of electrolysis, while the second 
deals with the conduction of the current through the liquid, the mecha- 
nism of conduction or of resistance, and its relation to other physical 
properties. In this no transformation of energy takes place but the 
frictional generation of heat. The secondary actions may in time affect 
the nature of the electrolyte, and the other effects at the electrode alter 
the magnitude of the current; but primarily the two parts of the subject 
are independent. 
Parr ITI. 
§ a.— What is an Electrolyte ? 
_ The complete answer to this question would imply the complete solu- 
tion of the problem of electrolysis, just as in the theory of light the 
complete solution is the answer to the question, What is common light ? 
_ Putting the question more definitely—What must be the physical 
state and chemical constitution of a substance in order that the conduc- 
tion of electricity through it may be attended with the decomposition of 
the substance into ions appearing only at the electrodes? 
In order to show that a particular substance is an electrolyte, the 
chemical decomposition produced by the current must be demonstrated 
either by the separation and exhibition of the products, or by the E.M.F. 
of polarisation. On account of tke sensitiveness of electrical instruments, 
the latter is the more delicate method; but the analogy between an 
electrolyte of high resistance and a leaky condenser is so close that the 
distinction between a dielectric and an electrolyte may sometimes be 
difficult to draw. , 
The liquids whose conduction is undoubtedly electrolytic vary very 
greatly in conductivity. To give an idea of the extent of the variation, I 
have compiled a rongh table of conductivities of a number of liquids, 
conductors and non-conductors (the numbers taken mainly from Wiede- 
mann’s ‘ Electricitit’). 
* Wied. Elec. 2, p. 780. 2 Wied. Zlec, 2, p. 783. 
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